Conventionally, as this kind of premixing apparatus, there is known in a Patent Document 1 a premixing apparatus comprising a fan for supplying air, and an air tube which is disposed on an upstream side of the fan so that air flows therethrough. Suppose: that a longitudinal direction of the air tube is defined as an X-axis direction; that, in the X-axis direction, the direction looking toward an upstream side of air flow is defined as an X-axis plus direction; and that the direction looking toward a downstream side of the air flow is defined as an X-axis minus direction. The premixing apparatus further comprises: a bottomed gas tube which is connected to a downstream end of a gas supply passage for supplying the fuel gas, an end part, in the X-axis minus direction, disposed in the air tube being closed; an air adjusting valve lying opposite to an air inlet port which is positioned at an end part, in the X-axis plus direction, of the air tube, in a manner to be movable back and forth in the X-axis direction to thereby vary an opening degree of the air inlet port; a gas adjusting valve lying opposite to a gas outlet port which is positioned at an end part, in the X-axis plus direction, of the gas tube, in a manner to be movable back and forth in the X-axis direction to thereby vary an opening degree of the gas outlet port; and a common actuator which drives in the X-axis direction the air adjusting valve and the gas adjusting valve.
It is to be noted here that the gas supply passage has generally interposed therein a zero governor which maintains the secondary gas pressure at atmospheric pressure. In this case, the fuel gas supply amount varies with the differential pressure between the atmospheric pressure that is the secondary pressure and the suction negative pressure of the fan. Then, since the suction negative pressure of the fan varies with the number of fan revolution, the fuel gas supply amount will vary with the number of fan revolution, that is, the air supply amount. Therefore, by controlling the number of fan revolution according to the required combustion amount, the amount of the air and the fuel gas according to the required combustion amount is understood to be supplied to the burner.
However, once the number of fan revolution has fallen below the lower-limit number of revolution at which the number of fan revolution can maintain the proportional characteristics of the air supply amount, it will no longer be possible to supply the amount of the air and the fuel gas according to the required combustion amount. Therefore, in the above-mentioned apparatus of the conventional example, in a region in which the required combustion amount falls below the predetermined value corresponding to the lower-limit value of the number of fan revolution, the opening degrees of the air inlet port and the gas outlet port are adjusted by the air adjusting valve and the gas adjusting valve in a state in which the number of fan revolution is maintained at the lower-limit number of revolution. The air and the fuel gas can thus be arranged to be supplied according to the required combustion amount below the predetermined value.
By the way, there is an example in which the gas supply passage has interposed therein a proportional valve in place of a zero governor. In this case, the air tube may be disposed in any of the upstream side and the downstream side of the fan. The proportional valve serves to supply the fuel gas in an amount proportional to electric current charged thereto (proportional valve current). The proportional valve current is controlled such that fuel gas can be supplied in an amount according to the required combustion amount. However, in case the required combustion amount falls below the predetermined value, and the proportional valve current has fallen below the lower-limit current at which the proportional valve current can maintain the proportional characteristics of the amount of gas supply, the fuel gas can no longer be supplied in an amount according to the required combustion amount. Accordingly, even in an example in which the proportional valve is interposed in the gas supply passage, in a region in which the required combustion amount falls below the predetermined value, while maintaining the number of fan revolution at the lower-limit number of revolution and also while maintaining the proportional valve current at the lower-limit current, the opening degrees of the air inlet port and the gas outlet port are adjusted by the air adjusting valve and the gas adjusting valve. It is thus possible to supply the air and the fuel gas in amounts according to the required combustion amount below the predetermined value.
By the way, in the above-mentioned conventional example, it is for the purpose of disposing the gas adjusting valve in the neighborhood of the air adjusting valve driven by the common actuator that the gas outlet port is disposed at the end part, in the X-axis plus direction, of the gas tube. This arrangement, however, will give rise to the following disadvantage. In other words, the effect of the dynamic pressure of the air that flows in from the air inlet port positioned at the end part, in the X-axis plus direction, of the air tube will be extended to the gas outlet port. As a result, the amount of fuel gas supply is likely to become unstable.